Abstract
Introduction
Using the transition of impurity levels in semiconductor materials,Blocked Impurity Band (BIB) detectors can cover the 5-30 μm band,with the cut-off wavelength of Si:Ga up to 20 microns [
Year | Institution | Cooling power | Input power | Detector | Cryocooler systems |
---|---|---|---|---|---|
1981 | LPI/RAS | 500 mW@4.4 K | 1.5 kW | BET-IM | Two-stage SC+JT |
2001 | JAXA-SHI | 23 mW@4.57 K | 166 W | - | Two-stage SC+JT |
2006 | NGST-ACTDP | 7.75 mW@4.5 K | 229 W | - | Three-stage SPTC+JT |
2007 | BALL-ACTDP | 75 mW@6.2 K | 300 W | - | Three-stage SC+JT |
2011 | NGAS-ACTDP | 113 mW@6.2 K | 104 W | JWST-MIRI | Three-stage SPTC+JT |
2014 | TIPC/CAS | 11.6 mW@4.5 K | 473 W | - | Three-stage SPTC+JT |
2021 | JAXA | 40 mW@4.5 K | 90 W | ATHENA-X | Two-stage SC+JT |
Table 1. Performance of 4.2 K cryocooler systems
The cryocooler at 4.2 K that is commonly used on the ground is the GM cryocooler,which has a large weight and an oil compressor that is unsuitable for space use. Both the multi-stage Stirling pulse tube cryocooler (SPTC) and the helium Joule-Thomson cryocooler (JTC) can achieve the BIB detector's required cooling capacity of 4.2 K [
As can be seen in
Besides,the stability of the operating temperature of the BIB detector has decisive significance for its high-performance infrared astronomical detection. This could be further attributed to the lower dark current and larger photoresponse at stable low temperatures. Due to the doping-induced bandgap narrowing effect and little impurity ionization energy,the sharp rise in dark current would be caused by even a tiny increase in temperature[
1 Design goals
In accordance with the long-term stability,low vibration,and deep cryogenic requirements of the large area array BIB detector,the first-stage Stirling cryocooler and the second-stage power recovery piston phase-modulated Stirling pulse tube cryocooler coupled with a helium JT technology system are proposed. A 300 mW@4.2 K JT cryocooler,which input power is @1.8 kW,is developed as a detector cryogenic component for the barrier impurity band (BIB). In comparison to the Soviet Union's 0.5 W@4.2 K cryocooler (complete machine 127 kg),the JT system,in conjunction with piston phase-modulation technology and high-frequency ultra-long stroke compressor technology,significantly reduces overall machine weight and achieves high cooling capacity in a lightweight cryocooler (weight 30 kg). This technical path has advantages in the development of a cryocooler with a large cooling capacity of 4.2 K,and its comprehensive indicators achieve international leadership status.
Parameters | Target values | Design values |
---|---|---|
Cooling capacity | ≥0.3 W@4.2 K | 0.5 W@4.2 K |
Power consumption | ≤1.8 kW | 1.6 kW |
Weight | ≤30 kg | 27 kg |
Temperature Fluctuation | ≤±2 mK(30 min) | ≤±2 mK(60 min) |
Table 2. Design specification
For this design,the cooling capacity should be a minimum of 0.3 W@4.2 K,the input power should not exceed 1.8 kW,and the weight better be less than 30 kg. When the cryocooler is designed with redundancy,the required input power is 1.6 kW. Due to the physical property deviation in the liquid helium temperature zone and the years of development experience of the unit,the cryogenic capacity is designed with more than 40% redundancy during thermal design,resulting in a cryogenic performance of 0.5 W at 4.2 K,as shown in
The design is depicted in
Figure 1.4.2 K JT cryocooler system
2 Design considerations of the 0.3 W@4.2 K lightweight cryocooler system
2.1 Pre-cryocooler
To achieve 0.3 W@4.2 K,a new JT cryocooler (JTC) pre-cooling system has been compared and selected. Combining the advantages of a two-stage precooler and JT technology,designing JTC uses the thermal coupling method to realize efficient coupling between the precooler and JT system,combining the advantages of the precooler and JT technology at different temperature ranges,which is one of the best ways for the space 4.2 K large cooling capacity application.
Under the performance target of 0.3 W@4.2 K,the temperature of the second-stage Stirling-type pulse tube cryocooler (SPTC) is 15 K and the high-pressure is 1.6 MPa. The required mass flow can be calculated by the temperature of second-stage of SPTC and high-pressure. Since the high-pressure has the best value,When the pre-cooling temperature of the second-stage pulse tube is determined,the mass flow will decrease first and then increase for the given second-stage SPTC temperature,with a minimum value. Combined with the minimum power consumption relationship,the mass flow of the helium JTC is 45 mg/s. According to the design requirements and the optimal parameter matching relationship,it can be concluded that the pre-cooling capacity of the two-stage precooler should not be less than 4 W@80 K and 0.8 W@15 K. Considering redundancy and radiation heat loss,the required cooling capacity of two-stage precooler is determined as 15 W@80 K and 0.9 W@15 K.
The structure of the two-stage precooler used in this system is shown in
Figure 2.Structure of the two-stage precooler
The use of AWDW in the second-stage could achieve lower temperature,higher performance and higher efficiency [
Because the gas stiffness is larger than the damping value,the gas force stiffness from the work-recovery chamber is opposite to the gas force stiffness from the expansion chamber. The gas stiffness in the expansion chamber can be significantly reduced by reasonable design [
Since the acoustic work at the hot end of the SPTC in the temperature range of 15 K is small. A thermally coupled two-stage SPTC has been developed which uses an active warm displacer (AWD) without work-recovery as a phase shifter,as shown in
Figure 3.Photo of 15 K second-stage pulse tube cryocooler
Through experimental tests,the second-stage SPTC can obtain the maximum cooling capacity at the second stage when the frequency is 30 Hz.
Figure 4.(a)Effect of frequency on 15 K rCOP,(b)Effect of frequency on 15 K cooling capacity
The single-stage SC is an integrated pneumatic structure with a free displacer. Its pneumatic displacer moves sinusoidally with the driving piston and maintains a certain phase difference with the compressor driving piston through structural design to achieve cooling capacity. The free displacer performs regular sinusoidal motion under the combined effects of the gas force,the spring force caused by the leaf spring,and the friction resistance. According to the stress analysis,the differential equation of motion vector can be written,as shown in
The theoretical cooling capacity of SC is equal to the expansion work. The expression is as follows (5):
The theoretical cooling capacity can be expressed as:
In addition,
A numerical simulation software is used to model and analyze. After the optimization of the regenerator structural parameters,the influence of the mechanical stiffness and the diameter of the push rod on the performance of the cryocooler is mainly considered.
Figure 5.Effect of mechanical stiffness on Stirling cryocooler
With an increase in the diameter of the push rod,the area difference between the two ends of the displacer increases,leading to the change in the displacer pneumatic force. Therefore,the displacement and phase of the pneumatic displacer are mainly affected by the diameter of the push rod. Thus,there are optimal diameters of the push rod to obtain maximum cooling capacity and performance efficiency. The smaller diameter of the push rod,the smaller the mass of the mover,which is beneficial for reducing the weight of the displacer. The calculated suitable rod diameter within the optimal range is around 7 mm,as this time the COP is approximately 13%,as we can see in
Figure 6.Influence of the diameter of the push rod on Stirling cryocooler
Within this cryogenic system,the vibration of the precooler is a crucial factor that requires our attention. This is due to low vibration is beneficial to the stable operation of the coupled BIB detector. Taking the first-stage Stirling cryocooler,which has relatively large vibrations,as an example,an active vibration damper has been adopted to reduce its vibration output and solve the problem. The basic principle of a vibration damper is the momentum balance method. Active vibration damping uses an electrically driven actuator to achieve momentum balance through active control. During operation,momentum balance is achieved by adjusting the driving phase and voltage amplitude between the damper and the compression piston. The technology has been successfully applied in multiple space projects by our research team. During the experiments,it was verified that the first-stage Stirling cryocooler's individual vibration output was less than 1N. Similar active vibration reduction technology can also be used for pulse tube compressors and valve linear compressors. Besides,by adding small vibration dampers to the system platform,low vibration output of the system can be ensured.
2.2 Valved linear compressor with high-pressure ratio and large mass flow rate
According to the decomposed indicators 0.5 W@4.2 K cryogenic performance: when the secondary pre-cooling temperature is 15 K,the valved linear compressor(VLC) needs to provide the JTC with a pressure of 0.1 MPa at the evaporator. It is preliminarily estimated that there is a pressure drop of about 10~20 kPa without the casing pressure drop loss. Considering that in practical application,the design margin given for the index parameters is 18%. The design demand index of VLC is 0.08 MPa for low-pressure,1.6 MPa for high-pressure,18.9 for total pressure ratio,53 mg/s for mass flow,40 Hz for operation frequency,and 15% for volumetric efficiency.
According to the existing structure and experiment,a double-piston opposed compressor structure is adopted and the valve group is external,which obtains the cooling capacity of 110 mW at 4.2 K. According to the decomposition index,the power consumption of VLC does not exceed 0.8 kW and the weight does not exceed 10 kg. That is to meet the performance while achieving the compactness and lightweight of VLC units.
The gas load force of the linear compressor is the combined force of the working gas on the front and rear faces of the piston [
In the VLC,the gas force is the only factor that produces the higher harmonic component in the mechanical vibration system. The higher harmonics are attenuated and can be approximated by the first harmonic component. The gas load force changes periodically and can be expanded into the Fourier series. The simplified gas load force is:
The gas load force can be decomposed into 1 equivalent gas spring,1 gas damping and 1 static force,and its expression is as follows:
The working process includes compression,exhaust,expansion,and suction.
During compression,the compressor chamber pressure increases and stops when it reaches
where the VLC displacement is:
where the phase angle of the compression phase
According to the thermal design,the first and second stage gas stiffness and its gas damping parameters are obtained. Combined with the mechanical damping and the leaf spring stiffness,the total stiffness and total damping are obtained. Maxwell is used to designing the motor and the miniaturization and optimization design of the motor can be carried out at the same time. By reducing the radial thickness of the inner pole,the outer diameter of the magnetic circuit is reduced,so that the motor current is close to the critical current value,which can minimize the motor volume and weight. By reducing the radial thickness of the inner pole,the trend of motor efficiency is obtained as follows in
Figure 7.Compressor lightweight selection
2.3 Low temperature JT heat transfer system
The key to realizing 500 mW@4.2 K is the design of a tube-in-tube heat exchanger with high efficiency and low-pressure drop. The fluid has a certain pressure drop in the high-pressure channel of the counter current casing heat exchanger at all stages and the reflux fluid in the whole low-pressure channel to varying degrees. Therefore,in the energy equation,it is considered that the enthalpy of the fluid is not only related to temperature but also a function of pressure [
For the low-pressure drop,we found that the pressure drop of the low-pressure channel in CFHX3 is about 200 Pa. For a helium JTC operating at 4.2 K,the pressure at its evaporator is usually about 100 kPa,and the pressure drops only accounts for about 0.2% of the low-pressure. In addition,the following calculation shows the effect of pressure drop on the theoretical cooling capacity. The theoretical cooling capacity of helium JTC can be calculated as [
where
The analysis of energy transfer and exchange between control bodies is very important: the energy transferred by high-pressure fluid to materials is
For a microelement,the momentum and energy equations of high-pressure fluid,low-pressure fluid and wall material are established respectively. The control equations are shown in
Momentum equation: the momentum equations are shown in equations (
Energy equation: the difference of the sum of enthalpy and kinetic energy between the fluid at the inlet and outlet is the energy absorbed or released by the fluid. The enthalpy is a function of temperature and pressure.
The calculation equations of pressure drop,heat transfer and of two-phase flow are shown in
Calculation range | Correlation | Applicable type |
---|---|---|
0<Re<3000 | Laminar flow | |
Re>3000 | Circular tube | |
150<Re<120000,0.5<Pr<2000 | Coil | |
Re>10000 | Homogeneous model of two-phase flow |
Table 3. Selected correlation expression and applicable scope
Figure 8.Effect of length on heat transfer efficiency
2.4 Comparison of previous experiments in 0.1 W@4 K
The design specifications state that the cooling capacity must be greater than 0.3 W@4.2 K (23 ± 5 °C),have an input power of less than 1.8 kW,and weigh less than 27 kg. Given the redundancy,the cooling capacity ought to be 0.5 W@4.2 K,and the input power ought to be less than 1.6 kW. The design of the system structure is depicted in
Figure 9.0.3 W@4.2 K JT cryocooler system structures design
We developed an experiment bench of the 0.1 W@4.1 K previous prototype of the cryocooler system,and its external structures are shown in
Figure 10.0.1 W@4.1 K JT cryocooler system experimental setup
Experiments on the previous 0.1 W@4.1 K prototype of the cryocooler system have validated the system's design method,as shown in
0.1 W@4.1 K Design value | 0.1 W@4.1 K Measure results | 0.3 W@4.2 K Design value | |
---|---|---|---|
Input power | 700 W | 647 W | 1800 W |
Cooling capacity | 150 mW@4.1 K | 110 mW@4.1 K | 500 mW@4.2 K |
Mass flow | 17 mg/s | 14.1 mg/s | 45 mg/s |
First-stage temperature | 83.9 K | 83.1 K | 80 K |
Second-stage temperature | 20.0 K | 17.5 K | 15 K |
Table 4. Comparison of system performance parameters
Once the cooling performance target of 0.5 W @ 4.2 K,a secondary pre-cooling temperature of 15 K and a high-pressure of 1.6 MPa have been determined. The required mass flow rate can be calculated,and the formula is as follows:
where
Figure 11.Relationship between second-stage temperature,high-pressure and mass flow rate at 0.5 W @ 4.2 K
When tripling the mass flow and input power of the JT system,and decreasing the pre-cooling temperature,the JTC system can reach 0.3 W at 4.2 K. Based on a new design concept,it will have three times the cooling capacity,indicating that it will have a substantial cooling capacity.
3·Interconnection with BIB detector
The fluctuating temperature of the helium JTC has a correlation with the change in ambient temperature,but the change in ambient temperature will also affect the performance of the cryocooler,resulting in the change of pre-cooling temperature at all levels. Consequently,the temperature fluctuations of the helium JTC are caused by environmental temperature fluctuations and are also influenced by the temperature fluctuations of the two-stage pre-cooling temperatures.
Epitaxial growth and ion implantation are two typical techniques to develop the BIB device. The epitaxial detector could be understood as the vertical detector. According to where the infrared signal illuminates,the vertical detector is divided into two architectures: back-illuminated (BSI) and topside-illuminated (TSI) architecture,as shown in Figure
Figure 12.(a) Structure of topside-illuminated (TSI) vertical BIB detectors,(b) structure of back-illuminated (BSI) vertical BIB detectors,(c) structure of ion-implanted BIB detectors,(d) detector band diagram under positive bias with thermal incidence,(e) electric field intensity as a function of position at different temperatures,(f) the relationship between the depletion region width with temperature.
When the temperature rises,the electrons originally bound around the impurity atoms are thermally ionized to the conduction band,which contributes a large thermal noise as shown in
The result of the 0.1 W@4.1 K experiment indicates that the temperature of the evaporator that provides cooling capacity for the BIB detector fluctuates by 6 mK in
Figure 13.Temperature fluctuations and control
4 Conclusion
By comparing with the previous experimental system of 0.1 W@4.1 K,an optimized cryogenic system is introduced for obtaining a large cooling capacity at 4.2 K with high efficiency. The efficiency and lightweight design are completed within 30 kg,and the input power is within 1.8 kW. We optimize and enhance each component of the cryocooler system:
(1) The performance of the two-stage pre-cooling system is consistent and its cooling capacity is substantial. It comprises a Stirling cryocooler for 80 K and a Stirling-type pulse tube cryocooler for 15 K. Adjusting the frequency achieves the maximum COP and cooling capacity of the second-stage pulse tube. By simulating the rigidity and rod diameter of the SC,the optimal combination can be determined.
(2) A double-piston opposed VLC is used in the JT cycle,to generate a pressure of 0.1 MPa. By reducing the radial thickness of the inner pole,the outer diameter of the magnetic circuit is decreased,bringing the motor current close to its critical value,thereby minimizing the motor's volume and weight. The VLC can get a total pressure ratio of 18.9 and a mass flow rate of 53 mg/s.
(3) For the JT cycle,the specific heat transfer process was analyzed. Then,we determined the optimal relationship between the length and the heat transfer coefficient,which will improve the performance of the heat transfer solution. Using the results of previous experiments,enhance the system's design to achieve a high cooling capacity of 0.3 W at 4.2 K and a lightweight design.
Through the integration design of the JT system,the cooling performance of 0.3 W@4.2 K with an input power of 1.8 kW can be obtained. To further improve the cooling performance,the optimal parameters to obtain the maximum cooling capacity under minimum input power need to be investigated in detail. The study can provide cryogenic technology guarantees for the coming space large array BIB detectors.
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